Something no one seems to have spotted is that from Volcanic Zone until Oil Ocean the Sonic life counter becomes a Tails one...

I haven't actually played it yet, so if there is a reason for this I appologize, but it does look strange when you control Sonic and it's Tails who has a life total...

That's because the game isn't overriding the defaults. By default there's a sequence Sonic-Tails-Miles (I think) which is overridden for levels which work. I don't know how to override it, but I'm sure there's a way.